Loi Krathong takes place on the evening of the full moon of the 12th month in the traditional Thai lunar calendar; hence, the exact date of the festival changes every year. In the Western calendar this usually falls in the month of November. In 2015 it was celebrated on November 25th; in 2016 it was celebrated on November 14th.

According to the Royal Institute Dictionary 1999, loi (ลอย) means "to float", while krathong (กระทง) has various meanings, one of which is "a small container made of leaves which to be floated on water during the Loi Krathong festival".[1]Krathong is traditionally small leaf container which are made to hold small portion of goods. Which, generally, are some kind very traditional Thai dish, such as "Hor Mok", and some Thai traditional dessert. The traditional krathong for floating at the festival are made from a slice of the trunk of a banana tree or a spider lily plant. Modern krathongs are more often made of bread or Styrofoam. A bread krathong will disintegrate after a few days and can be eaten by fish. Banana stalk krathong are also biodegradable, but Styrofoam krathongs are sometimes banned, as they pollute the rivers and may take years to decompose. A krathong is decorated with elaborately-folded banana leaves, incense sticks, and a candle. A small coin is sometimes included as an offering to the river spirits.

On the night of the full moon, Thais launch their krathong on a river, canal or a pond, making a wish as they do so. The festival may originate from an ancient ritual paying respect to the water spirits.

Government offices, corporations, and other organizations launch large decorated krathongs. There are competitions for the best such krathong. A beauty contest is a regular feature and fireworks have become common in recent years.

Loi Krathong is often claimed to have begun in the Sukhothai by a court lady named Nopphamat. However, it is now known that the Nopphamat tale comes from a poem written in the early Bangkok period.[2] According to King Rama IV, writing in 1863, it was a Brahmanical festival that was adapted by Thai Buddhists in Thailand to honor the Buddha, Prince Siddhartha Gautama. The candle venerates the Buddha with light, while the krathong's floating symbolizes letting go of all one's hatred, anger, and defilements. People sometimes cut their fingernails or hair and place the clippings on the krathong as a symbol of letting go of past transgressions and negative thoughts. Many Thais use the krathong to thank the Goddess of Water, the Hindu Goddess Ganga, Phra Mae Khongkha (Thai: พระแม่คงคา).

The beauty contests that accompany the festival are known as "Nopphamat Queen Contests". According to legend, Nang Nopphamat (Thai: นางนพมาศ; alternatively spelled as "Noppamas" or "Nopamas") was a consort of the 13th century Sukhothai kingSri Indraditya (who is also known as Phra Ruang) and she had been the first to float a decorated raft.[3] However, this is a new story which was invented during the first part of the 19th century. There is no evidence that a Nang Nopphamat ever existed. Instead, it is a matter of fact that a woman of this name was the leading character of a novel released at the end of the reign of King Rama III, around 1850 CE. Her character was written as guidance for all women who wished to become civil servants.

Kelantan in Malaysia celebrates Loi Krathong similarly, especially in the Tumpat area. The ministry in charge of tourism in Malaysia recognises it as an attraction for tourists. Many people visit the celebration each year.

Lo Krathong coincides with the Lanna (northern Thai) festival known as Yi Peng (Thai: ยี่เป็ง). Yi means "two" and peng means a "full moon day". Yi Peng refers to the full moon day in the second month according to the Lanna lunar calendar (the twelfth month according to the Thai lunar calendar).[4]

Swarms of Lanna-style sky lanterns (khom loi (Thai: โคมลอย), literally: "floating lanterns") are launched into the air where they resemble large shoals of giant fluorescent jellyfish gracefully floating through the sky. The festival is meant as a time for tham bun (Thai: ทำบุญ), to make merit. Khom loi are made from a thin fabric, such as rice paper, stretched over a bamboo or wire frame, to which a candle or fuel cell is attached. When the fuel cell is lit, the resulting hot air is trapped inside the lantern and creates enough lift for the khom loi to float up into the sky.

Because they are a hazard to passing aircraft and "...can cause damage to important places in the areas such as the Grand Palace [sic], temples and governmental offices,..." khom loi are increasingly subject to governmental restrictions. In Bangkok in 2014, revellers are prohibited from launching floating lanterns in all event areas from 18:00 on 6 November to 05:00 the next day.[5] Violators may face execution or a life sentence or serve a lighter sentence of 5 to 10 years in prison, if damages are minor. Offenders are also guilty of violating Section 232 of the Criminal Code and that alone carries a sentence of 6 to 7 years in prison and a fine up to 1,000 to 14,000 baht.[6]

During the festival, people also decorate their houses, gardens, and temples with khom fai (Thai: โคมไฟ), intricately shaped paper lanterns which take on different forms. Khom thue (Thai: โคมถือ) are lanterns which are carried around hanging from a stick, khom khwaen (Thai: โคมแขวน) are the hanging lanterns, and khom pariwat (Thai: โคมปริวรรต), which are placed at temples and which revolve due to the heat of the candle inside. The most elaborate Yi Peng celebrations can be seen in Chiang Mai,[7] the ancient capital of the former Lanna kingdom, where now both Loi Krathong and Yi Peng are celebrated at the same time resulting in lights floating on the waters, lights hanging from trees/buildings or standing on walls, and lights floating in the sky. The tradition of Yi Peng was also adopted by certain parts of Laos during the 16th century.

The day after Loi Krathong, the Bangkok Metropolitan Administration (BMA) cleaned six tonnes of rubbish from the city's waterways. The city governor said that 661,935 floats were collected from waterways across Bangkok. Of these, 617,901 (93.7 percent) were made of decomposible natural materials, while 44,034 were non-biodegradable Styrofoam floats. There were 163,679 fewer krathong collected than in 2015. The city mobilized 210 workers and 45 boats to collect floats from the Chao Phraya River and canals.[8]